Optical signals are transmitted via optical conductors. They are frequently amplified by using fiber amplifiers. These use either specially doped lengths of fiber, or utilize nonlinear effects on normal transmission fibers, as in the case of the Raman fiber amplifier described in ntz, volume 43, (1990), issue 1, pages 8 to 13.
In the case of many transmission devices, use is also made of attenuators with the aid of which required level values (e.g., the input levels of amplifiers) are adjusted, as is described, for example, in IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 6, No. 4, April 1994, pages 509 to 512.
Modern transmission systems use a plurality of signals with the aid of the wavelength division multiplex method, (WDM), in which a plurality of transmission channels are combined in each case to form a transmission band that is amplified in common. The Raman effect produces an influence between the transmission bands where the levels of the individual signals (channels) are affected differently, which is denoted as tilting and to date has mostly been compensated by nonlinear amplifiers and filters. The fundamentals of the stimulated Raman scattering are described in Nonlinear Fiber Optics, Second Edition, Govind P. Agrawal, Academic Press, Chapter 8.
European Patent Application EP 0 139 081 A2 describes an optical communication system in which the transmitted optical signal is amplified on the basis of the stimulated Raman effect by a plurality of pump signals of different wavelengths. The different pump signals are selected such that the gain characteristic and the signal levels run as ideally as possible.
European Patent Application EP 0 734 105 A2 discloses a fiber amplifier which is used by means of a pump signal and a mirror for the purpose of compensating the dispersion. FIG. 47 shows the tilting of the signal levels (slope gain) as a function of the pump power.
GB 2 294 170 A describes an arrangement for amplifying which monitors the number of active channels and also keeps the level at a preselected value in the event of absence of individual channels.
Patent Abstracts of Japan publication number 59065828/application number 57176312 describes an amplifier for a continuous optical signal (constant wave). The light from a signal source 11 of shorter waves is converted to the wavelength of the auxiliary light source of longest waves with the aid of the auxiliary light sources 13 to 20, tuned to the Stokes wavelength, on the basis of the stimulated Raman effect.
None of these references provides satisfactory adjustment or compensation of the tilting, in particular in the case of WDM systems with a plurality of transmission bands.
Particularly in the case of WDM systems in which a plurality of groups of signals are transmitted, the stimulated Raman scattering, (SRS), is to amplify the signals transmitted in “longwave” channels at the expense of the signals transmitted in “shortwave” channels. In other words, energy is extracted from the shortwave “blue” channels, which are more strongly damped with decreasing wavelength (increasing frequency), while this benefits the longer wave “red” channels. The larger the wavelength, the more the corresponding transmission channels profit. A similar statement holds for the spectral components of signals with high bit rates.
The influence of the SRS effect is illustrated in FIGS. 1 and 2. The left-hand diagram of FIG. 1 shows a constant reception level, independent of wavelength, of the blue transmission band (wavelength region) λB. The right-hand diagram illustrates the reception level when simultaneous use is made of a further “red” wavelength region for optical signal transmission. The smaller the wavelength of the blue transmission band, the stronger the attenuation.
The level relationships for the “red” transmission band λR are illustrated in FIG. 2. The left-hand diagram in FIG. 2 shows the linear level characteristic for a case where signals are transmitted only in this transmission band. If, in addition, there is transmission in the “blue” wavelength region, the level is raised higher with increasing wavelength. This depends only slightly on whether the signals in the transmission bands are transmitted in the same or opposite directions (co-propagating waves - counter-propagating waves). The change in the levels, illustrated in the right-hand diagrams of FIGS. 1 and 2, as a function of the wavelength, which corresponds to a pivoting about a common fulcrum, is denoted as tilting.
In today's typical transmission with two times eight channels, the effect described gives rise to additional attenuations or amplifications in a transmission section (approximately 40–80 km) of between 0.4 to 0.7 dB. In the case of transmission links with up to 10 or more transmission sections and a corresponding number of repeaters, these changes in level add up correspondingly. If one of the transmission bands is absent, there is also a very quick change in the level of the signal in the intact transmission band. The automatic gain control at the receiving end can usually not compensate these level fluctuations quickly enough, the result being error bursts in the millisecond region. A quick restitution of the previous level is required in this case.
For many applications, it is possible for the level and the tilting of signal bands to be adjusted frequently independently of one another.